1. Field of the Invention
This invention relates to a process for producing metal powder particularly with low oxidation and having a desired carbon content from a molten metal by virtue of the atomization method.
2. Description of the Prior Art
The production method of metal powder by the atomizing technique may be divided roughly into the gas-atomization method in which a gas is used as an atomizing medium and the water atomization method in which water is employed as an atomizing medium. The former method has the merit that, owing to the use of an inert gas or reducing gas, less oxidized metal powder may be obtained. However, due to the small cooling capacity of a gas, the cooling of powdered metal takes place slowly and powder particles may thus be prilled into spheres in the course of their cooling under the influence of surface tensions. Spherical powder is poor in mechanical strength after pressing and sintering, and is thus not preferred as a starting material for powder metallurgical products.
On the other hand, the water-atomization method features a high cooling speed and powdered metal particles have good sinterability and non-uniform shapes. Such powdered metal particles however require a reduction treatment prior to their use because they have been oxidized by oxygen contained in water or generated by the decomposition of water. The reduction treatment is carried out, mainly, by using hydrogen gas. However, equipment for the reduction treatment is expensive and the operation cost thereof is enormous, leading to the drawback that the prices of powdered products will become higher.
As a method capable of solving the above-described drawback of the water-atomization method, has been developed an atomization method using an oil or the like as the atomizing medium, namely, the so-called oil-atomization method. According to this method, the oxidation of metal powder, which takes place during its atomizing period, can be prevented almost completely. However, the oil-atomization method generally requires a decarburization treatment since oils are decomposed upon contact with a molten metal of a high temperature during their atomization and the carburization of metal powder takes place, thereby generally making a decarburization treatment necessary.
As described above, conventional atomization methods using a gas or liquid as an atomizing medium are accompanied by various problems.